a) Field of the Invention
The invention relates to a capacitive sound transducer comprising a diaphragm and a counterelectrode which is disposed at a short distance from the diaphragm and provided with first perforations. The invention further relates to a condenser microphone provided with a capacitive sound transducer according to the invention.
b) Description of the Related Art
A capacitive sound transducer of a condenser microphone contains a planar diaphragm which is moved by sound, and a perforated counterelectrode parallel thereto at a short distance therefrom. The diaphragm and counterelectrode are designed to be electrically conductive and form an electrical capacitor whose capacitance is dependent on the diaphragm deflection caused by the sound. Such a condenser microphone is known form DE 19715365, for example.
Due to the viscosity of the air, the narrow, air-filled space between the diaphragm and the counterelectrode, called the air gap, acts as a frictional resistance which inhibits movement of the diaphragm. This effect is used to control the movement of the diaphragm. However, the air gap resistance is not constant, but depends on the momentary distance between the diaphragm and the counterelectrode. When the diaphragm moves towards the counterelectrode, the air gap narrows, and as a result the frictional resistance becomes greater, otherwise smaller. For this reason, any over-pressure in front of the diaphragm that moves the diaphragm towards the counterelectrode will generate a smaller diaphragm deflection than an equally large under-pressure that moves the diaphragm away from the counterelectrode. For this reason, the movement of the diaphragm and the change in capacitance produced as a result is not a linear copy of the sound signal, but is nonlinearly distorted.
The degree of nonlinearity can be reduced by decreasing the diaphragm deflection by means of suitable measures, for example by stronger air-gap attenuation. However, this gives rise to disadvantageous effects because the transducer sensitivity is reduced, as a result of which the noise characteristics of the microphone are also detrimentally affected.
One advantageous option for reducing the nonlinearity of the diaphragm deflection is provided by the “symmetrical push-pull converter”, as described in DE 43 07 825 A1, for example. It contains a second counterelectrode with properties identical to those of the first counterelectrode and which is disposed in front of the diaphragm in such a way that similar air gaps are formed on both sides of the diaphragm. In this case, the movement of the diaphragm causes opposite changes in resistance in the two air gaps, which mutually compensate each other. By this means, the movement of the diaphragm is linearized and the transducer distortions are minimized.
In push-pull converters, the change in capacitance between the two counterelectrodes and the diaphragm is generally evaluated by applying the HF principle, by connecting both counterelectrodes to the electric circuit of the microphone. The disadvantage this involves, namely that the additional counterelectrode disposed in front of the diaphragm is directly exposed to humidity, with the result that its electrical insulation can be weakened, does not have an effect when the HF principle is applied, because said principle results in very low electrical impedances.
In the case of condenser microphones and electret microphones operating according to the NF principle, electrical operation of the front counterelectrode would then lead to substantially greater moisture sensitivity due to the very high electrical impedances that then arise. Until now, this disadvantage has stood in the way of the push-pull principle being applied to these types of microphone.
Another disadvantage of the capacitive sound transducers used in known condenser microphones is that, in those regions lying opposite the perforated regions of the counterelectrode, the diaphragm produces partial natural oscillations at high frequencies, and these oscillations lead to undesired, frequency-dependent changes in the transmission characteristics of the condenser microphone. The frequencies at which partial oscillations occur are dependent on the mechanical tension of the diaphragm and on the size and shape of the counterelectrode perforations. In many cases, they are within the frequency transmission range, that is the specified operating frequency range, and lead to undesired frequency-dependent changes in the transmission characteristics of the condenser microphone.
This undesired oscillation behavior at high frequencies can be sufficiently suppressed in those regions of the diaphragm which lie opposite the non-perforated regions of the counterelectrodes, if the distance between the diaphragm and the counterelectrode is made so small that the viscosity of the air in the air gap formed by the diaphragm and the counterelectrode ensures sufficient attenuation of diaphragm movements. However, this attenuation is absent in those diaphragm regions which lie opposite the counterelectrode, with the consequence that the undesired natural oscillations of the diaphragm are not suppressed.
Known methods for attenuating diaphragm movements, for example by means of a porous layer of fabric attached to the rear side of the counterelectrode, are unable to achieve sufficient attenuation of the partial oscillations because, at high frequencies; sufficiently direct action is prevented by the acoustic resilience of the air trapped in the perforated regions of the counterelectrode.
U.S. Pat. No. 4,817,168 discloses a directional microphone in the form of a condenser microphone, in which a diaphragm is arranged at a small distance from a counterelectrode provided with perforations. Said patent also discloses an air chamber which is separated from the counterelectrode and an intermediate wall with openings.
A condenser microphone provided with two conventional diaphragm-counterelectrode systems, which are separated by a solid body with a connecting channel, is known from GB 921,818.
A condenser microphone in which two perforated plates are arranged at a distance from each other with their perforations offset from each other, and which are provided with an attenuation layer is known from DE 821 217.